Electron discharge tube



March 1, 1949. w. G. TAYLOR ELECTRON DISCHARGE TUBE Filed May 5, 1946 IN V EN TOR, WE r1922 a- B Y Z. P WW ATTORNEY Patented Mar. 1, 1949ELECTRON DISCHARGE TUBE Warren G. Taylor, Essex Fells, N. J., assignorto Federal Telephone and Radio Corporation, New

York, N. Y.,

a corporation of Delaware Application May 3, 1946, Serial No. 666,972

Claims.

This invention relates to vacuum tubes of the ultra-high frequency type,especially those known H as Lighthouse tubes, in which the electrodesare spaced a minute distance apart to provide an extremely short transittime of travel of the electrons between the electrodes. The inventionrelates more particularly to a mountin arrangement assuring accuratespacing between the oathode and grid electrodes of such tubes.

The principal object of the invention is to provide a mountinarrangement for the cathode and grid which will permit an extremelyclose spacing between these electrodes without incurring variations inspacing due to heating or other conditions.

Vacuum tubes capable of handling large amounts of power have beendeveloped for use i at ultra-high frequencies, in which the electrodesare spaced extremely close together to create an extremely small time oftransit of the electrons in their path of travel. Such tubes withparallel plane electrodes have been commonly called Lighthouse tubes.They ordinarily comprise a cathode, grid and anode, and have been usedfor example, as oscillators and amplifiers. The electrodes have usuallybeen built into a resonant cavity system, there being, for example, aresonant cavity for the grid-cathode circuit, and another cavity for thegrid-anode circuit.

Owing to the fact that the grid is placed at only a minute distance fromthe cathode in such tubes, for example, the order of .005", it has beenan important problem in the construction and arrangement of theseelectrodes, to provide suitable means for providing and maintainingaccurately the spacing inasmuch as a very slight change of spacingduring operation due to heating or other conditions will produce anundesired change in the tube characteristics. A difficulty heretoforeencountered in the mountings of such electrodes has been the unevenexpansion of the material with heat, which undesirably changes thespacing between the cathode and grid.

According to my invention, I provide means for mounting the grid andcathode in relation to each other so that the spacing between them canbe accurately established without variation. I do this by attaching thegrid to a grid support, and providing a cathode by covering a surface ofan insulating support facing the grid, with an electron emissivesubstance. For the purpose of providing the desired critical spacingbetween the grid and cathode, I provide on the cathode support, a raisedrim or spacing member having a surface which engages the grid support.Preferably the cathode support has a cup-like shape with the rim of thecup held against the grid support and the cathode supporting surface israised from a central location of the cathode support. By accuratedimensioning of the cathode support in relation to the grid and gridsupport, there will be provided the desired minute spacing between thecathode and grid.

I prefer to make the cathode support of a ceramic material or glass,capable of withstanding heat and with as low a temperature coeiiicientof expansion as possible. By making the ceramic structure or high silicaglass of low temperature coefficient of expansion, the tendency forvariation of distance between the cathode and grid is practicallyeliminated, and the electrodes will accurately maintain their spacingthrough variations in temperature. Moreover, such tendency as may existfor heat expansion to occur will be selfcompensating owing to the shapeof the cathode support.

In order to provide for heating the cathode, the member on which thecathode material is placed is preferably provided with a recess at theside opposite the grid, and a cathode heating element inserted in therecess.

The foregoing and other features of my invention will be betterunderstood from the following detailed description and the accompanyingdrawings of which:

Fig. 1 is a perspective cross-section view taken at line l! of Fig. 2,showing a grid-cathode mounting arrangement according to my invention;

Fig. 2 is a top view of the arrangement shown in Fig. 1;

Fig. 3 shows the grid used in the arrangement of Figs. 1 and 2;

Fig. 4 is a cross-section of the grid taken at line i-fi of Fig. 3;

Fig. 5 is a top view of a rin or washer used for holding the grid in theconstruction of Figs. 1

and 2;

Fig. 6 is a cross-section view taken at line 6-6 of Fig. 5; and

Fig. 7 shows a grid-cathode structure mounted accordin to my inventionand included in a vacuum tube.

Referring to Figs. 1 and 2, there is provided a grid i which may beconstructed as shown in Figs. 3 and 4 of the usual grid wires 2,fastened within a suitable holder 3 which may be annular in shape, Forthe purpose of holdin the grid there are provided a pair of annularmounting washers or rings 4 and 5, each of which is constructed as shownin Figs. 5 and 6.

Referring to Figs. 5 and 6, the washer comprises the peripheral portion6 having a circular cutout portion 1 at its inner edge, providing ashoulder 8. In Fig. 1, two of these washers are placed together withtheir cutouts I placed against each other, providing the annular spacebetween the washers into which the supporting holder 3 ofthe grid snuglyfits so that it is held between surfaces 1 and abuts shoulders 8. Thegrid may, of course, be attached to one or the other of the washers inany other suitable manner, as may be desired.

There is provided a cathode supporting means in the form of acup-likemember'il of ceramic material or high temperature glass having aflat base portion l and a circular peripheral rim ll upstanding from thebase portion, and providing a fiat planar surface l2 at the top of thelip which engages the flat undersurface of the hollow washer 5.

There is centrally formed fromthe base portion within the cup, anupstanding cylindrical portion l3'having a fiat circular top l4. On thiscircular topl l there is deposited in a suitable manner a cathodiccoating I5 which may for example, be a deposition of metal providing anactive electronemissive surface such as the oxides of barium andstrontium.. This cathode is located centrally with reference to the gridand is provided with leads The dimensions of the circular peripheral lipportion l l relative to the central cylindrical portion I3 andthe-cathode layer on top of it are so related to each other that whenthe planar surface l2 of the circular lip is held directly against thewasher 5, the spacing between the upper surface of cathode l5 will bethe desired minute distance from the lower surface of grid I. Thisdistance may, for example, be as close as .005.

In order to guard against appreciable variation between cathode andgrid, I make the cup-like member 9 of an insulating material having avlow temperature coefiicient of expansion, so that subto that of Figs. 1and 2 mounted into a vacuum tube, of which only a part of the tube isshown. Sufficient of the tube is illustrated, however, to show therelative position of the cathode and grid to the anode 23, the anodebeing in the form of a solid cylindrical member having a diametersubstantiallythe same as that of the cathode l5 and located c'oaxiallywith the cathode. The grid is situated between the cathode and anode, as.1 Wu. The lower surface 24 of the anode is a plane fiat surfaceparallel to the surface of the grid and of the cathode. The anode isvery closely spaced to the grid, although it will not ordinarily bespaced quite as close as the cathode is to the grid. The spacing betweenthe grid and anode may, for example, be in the order of .020.

The anode may be held in relation to the grid in a suitable manner, forexample, by the metallic flange element 25, which is fastened to ashoulder of the anode and extends laterally. A glass section 21is-sealed to flange 25 and also to an annular flange 28 which is boltedor otherwise suitably attached or sealed to grid washer 5, as shown.

If desired, the annular cavity l'l between the rim H and the centralportion l3 of the cathode support, andalso the cavity 29 between thegrid and the flange 25 may form one vacuum chamber; and this may be doneby sealing the metal ring 22 to the metal washer 5i where they join. Insuch case, the chambers II and 29 may be evacuated in any suitablemanner, for example, through a central bore extending through the anodemember and providing an opening into cav ity 29 (not shown).

In order to support the electrode structure, there is shown a glasssupporting bowl 3!] which is sealed around its upper rim to the ring 28and has the cathode heater leads l9 and 20 sealed stantially no changein its shape'or dimension I occurs when the structure is heated. By alow temperature 'coefficient of expansion within the meanin of thisspecification and the appended claims, I mean a temperature coefficientless than 1x l0 Such a low temperature coefficient will givethedesired'freedom from excessive dimension changes. A suitable glass forthe purpose is, for example, one having predominantly one of thefollowing compositions:

1. 96% S102, 3% B203, 0.4% R2O3+RO2(A12O3) 2. Pure fused SiOz 3. A1203(Alundum) is placed in recess l6 and terminal leads l9 and 20 are takenfrom the filament. In order to prevent'the filament from burning out,the recess It should be evacuated, and a glass seal 2| is provided formaintaining this'vaicuum.

The glass cup may be held against the grid washer in any suitablemanner, for example, by,

screwing means or nuts and bolts, or other suitable means. In Figs. 1and 2, spring clamps 22 are shown clamping the elements together.

Fig. '7 shows a mounting arrangement similar through it.

It will be understood that the tube elements shown associated with thegrid and cathode mounting arrangements in Fig. '7 is only for purposesofillustration, and should not be considered as limiting in any way. Thegrid-cathode mounting arrangement according to my invention may beincorporated in any desired form of tube or arrangement, as may bedictated by individual circumstances.

Suitable cavities may conveniently be formed byattaching a cylindricalmember to the gridcontacting ring 28 and extending the cylinder ineither direction from the ring so as to form a coaxial cavity with theanode which can readily beadjusted by a suitable plunger arrangement,and a similar cavity with a cylindrical member which may be attached tothe cathode.

It will be recognized that the grid-cathode mounting arrangementaccording to my invention is'useful with many forms of tube and resonantcavity arrangements, and that the advantage attending the use of themounting will be utilized in whatever circuit arrangement is in use.

Owing to the low temperature coefficient of the ceramic cup-likemounting device 9, there is substantially no change in its dimensionswhen heated, and accordingly, very little variation in spacing betweenthe cathode and grid. Furthermore such tendency toward change ofdimension as may 'exist will be mostly'compensated because of the factthat the upstanding central member i3 is substantially the same lengthas the upstanding rim; and accordingly any change of dimension would bepractically the same in both the upstanding members. This would resultin maintenance of the prescribed spacing between the grid and cathode.

I claim:

1. A grid and cathode assembly for use in vacuum tubes comprising adisk-shaped grid having a plane surface, an annular washer holding thegrid within the Washer, a cup-shaped member of refractory insulatingmaterial havin a base portion and a circular rim with a peripheralsurface in contact with the surface of the washer, and an upstandingcentral portion protruding from the base portion toward the grid, saidupstanding central portion having a plane surface facin the grid, anelectron-emissive cathode covering the plane surface, and means makingelectrical connection with said cathode covering.

2. A grid and cathode assembly for use in ultra-high frequency vacuumtubes comprising a disk-shaped grid having a plane surface, an annularwasher holding the grid at its periphery, a

cup-shaped glass member having a base portion, a circular rim with aperipheral surface in con-- tact with a surface of the washer and anupstanding central portion protruding from the base portion toward thegrid, said upstanding portion having a plane surface facing the grid, anelectron-emissive cathode covering the plane surface and placed veryclose to the grid, said upstanding portion having a recess within it,and a cathode heating element within the recess.

3. A grid and cathode assembly for use in Vacuum tubes adapted. tooperate at ultra-high frequencies, said assembly comprising a grid electrode supported around its periphery by a holding element, a ceramicelement comprising a base portion and a centrally located upstandingportion providing a surface facing the grid and conforming with theshape of the grid surface, an electron emissive cathode on said surfaceand spaced close to the grid, said ceramic member having an upstandingportion around the central upstanding portion and having aperipheralsurface held against a surface of the grid-supporting member, thesurface of the cathode being spaced at a minute distance from thesurface of the grid, the annular space within the ceramic member betweenthe peripheral portion and the central upstanding portion providing acavity, and a recess in the upstanding portion, and a cathode heatingelement within said recess.

4. An assembly according to claim 3 in which the ceramic member is aglass having a low temperature coefficient of expansion.

5. A grid and cathode assembly for use in vacuum tubes adapted tooperate at ultra-high frequency, said assembly comprising a thin fiatgrid, a pair of washers holding the grid between them, a cup-shapedglass member having a base portion, a circular rim portion extendingfrom the base portion and having a peripheral surface in contact with asurface of one of the washers, and an upstanding cylindrical centralportion protruding from the base portion and providing an annular spacebetween the central portion and the peripheral portion, said upstandingportion having a surface facing the grid and a cathode placed on saidsurface at a minute distance from the grid, and means holding theperipheral surface of the rim portion against the washer.

G. A grid and cathode assembly for use in a vacuum tube adapted tooperate at ultra-high frequencies and providing relatively short transittime of electron travel between the cathode and grid, comprising adisk-shaped grid having a plane surface, an annular washer to which thegrid is attached, a cup-shaped glass member having a low temperature'coefiicient of expansion, said glass member having a base portion andan upstanding cylindrical portion providing a plane surface parallel tothe surface of the grid and facing the grid, an electron-emissivecathode layer on said fiat surface, and a circular rim portionupstanding from the base portion and having a peripheral surface adaptedto make contact with a surface of the washer, and means clamping saidperipheral surface and washer surfaces together, whereby the surface ofthe cathode is held spaced at a minute distance from the surface of thegrid and said minute spacing is accurately maintained.

7. A cathode assembly for use in vacuum tubes comprising supportingmeans of ceramic material of low temperature coefiic-ient of expansionincluding a base portion and an upstanding cylindrical portion having aflat top, said cylindrical portion being provided with a central borehaving an opening at the side opposite said fiat top, a coating ofemissive material on said flat top, said coating having leadselectrically connected thereto, and a heating filament arranged in saidcentral bore adjacent said fiat top and having terminal leads arrangedthrough said opening.

8. A cathode assembly for use in vacuum tubes comprising supportingmeans of ceramic material of low temperature coeficient of expansionincluding a base portion and an upstanding cylindrical portion having aflat top, said cylindrical portion being provided with a central borehaving an opening at the side opposite said fiat top, a coating ofemissive material on said flat top, said coating having leadselectrically connected thereto, a heating filament arranged in saidcentral bore adjacent said flat top and having terminal leads arrangedthrough said opening and a glass seal closing said opening, said centralbore being evacuated.

9. A cathode assembly according to claim 8 in which said emissivecoating is barium oxide.

10. A cathode assembly according to claim 8 in which said emissivecoating is strontium oxide.

WARREN G. TAYLOR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES "PATENTS Number Name Date 1,913,978 Ewen June 13, 19332,414,137 Branson Jan. 14, 1947

